1. Field of the Invention
The present invention relates to a radio transmission system comprising at least one radio apparatus which comprises a receiving arrangement with at least two receiver front-ends which are coupled to an array of at least two antennas and combining means so as to form beams. Such a system can be a mobile radio system, a cordless telephony system, a paging system, a car radio broadcast system, a GPS (Global Positioning System), an in-house audio link system or home-cast system, wireless headphones, or any other radio transmission system.
The present invention further relates to a radio apparatus for use in such a system.
2. Discussion of the Related Art
A radio transmission system of this kind is known from the European Patent Application EP 0 459 038, in which off-line processing is applied to process signals from a phased-array of antennas for optimising the reception of a desired signal in the presence of interfering signals. In the known system coefficients of tapped delay lines as phase shifting networks are computed and updated so as to find an optimum for a received signal quality. The method as applied in the known system is a complicated brute-force method which is not suitable in environments where reception conditions vary quickly. Such a brute force method is difficult to implement in e.g. a low power portable radio apparatus because a lot of processing capacity is required, requiring considerable battery power. Furthermore, when applying the known method, it cannot be guaranteed that a global optimum is found. In the European patent application EP 0 308 830 another known system is described in which a direction of arrival of incoming radio waves is detected, i.e. the direction of incoming radio waves having the maximum intensity among all the incoming radio waves arriving in all directions. The incoming radio waves are supplied via an array of antennas to a number of receiver front-ends, and via FFT (Fast Fourier Transform) processing means such a maximum intensity direction is determined. By adjusting phase shifters in receiving branches the direction of reception can be changed. In the European Patent Application EP 0 610 989, another known radio transmission is described, in which radio waves are received via at least two antennas. Before the received signals are fed to a receiver, a phase difference between the radio signals as received by the antennas is determined. Such a phase difference contains information as to the direction from which was sent. In EP 0 308 830 and EP 0 610 989 bi-directional transmission systems are described in which radio waves are transmitted in a detected optimum reception direction. The methods applied in the known systems, whether uni-directional or bi-directional, are not optimal for a multipath environment in which signal absorbing or reflecting objects such as buildings or strong interference sources are present in reception paths. When applying the information from the thus determined optimum reception direction in a bi-directional transmission for transmitting in the opposite direction, this could even lead to a transmission in a wrong direction, e.g. into the direction of an interference source or into the direction of an absorbing object so that at the opposite reception side a poor quality signal or no signal at all is received.